can I use this, or this there one I can use to build a horizontally slattex gate?

This gate has slots to mount three horizontal boards on which you would nail vertical pickets. To hang horizontal pickets or other horizontal pieces, you would need to nail three vertical furring strips to the horizontal boards installed inside the gate frame. Your horizontal fencing would stick out the width of the furring strips.

I am unsure what a "slattex" gate is, but there are several gate frame widths. A small size that adjusts up to 4 feet wide and 4 feet tall. The larger one extends out to 5 feet wide and 6 feet tall. There is about 12 inches of width adjustment, but the height is not adjustable so the correct height frame must be selected.

As per the manufactures instruction the Adjust-A-Gate recommends placing the gate boards vertical so it will matches the desired height of the fence. While horizontal fence boards is not recommended, we have no instructions for such placement, It can be done. You would have to use self-tapping screws. Screw the fence boards into the frame itself. To help extend the height to match the fence itself, you may want to attach a 2x2 or 2x3 piece of wood on the inside of the Adjust-A-Gate frame to match the fence length top and bottom. Again we only recommend manufactures instructions. How the wood boards react to a horizontal placement we do not know.